The first explicit mention of the heart that we found is when the boy comes into the library asking Neil to direct him to the art section. Neil thinks he says the "heart section" and we get the feeling that there are matters of the heart in Goodbye, Columbus beyond the love affair between Brenda and Neil.
The play on words of "art" and "heart" might suggest that a person's art is what is in a person's heart. For Neil, this seems to mean the library and books, while for Brenda it might be sports or whatever it is she's studying in school (if Neil knows or cares what her major is, he doesn't say). For Mrs. Patimkin, perhaps it's her practice of Judaism and doing good works. For Ron, it could be his perfect basketball game, and for Mr. Patimkin, it might be business.
And that brings us to another important "heart" in the story, Patimkin Kitchen and Bathroom Sinks. As we discuss in "Setting," Patimkin Sinks is located "in the heart" (6.218) of a very poor area of Newark now mostly inhabited by poor black people, but previously by poor Jewish immigrants. Patimkin Sinks is also the "heart" of the Patimkins's wealth and, therefore, their suburban life. Without Patimkin Sinks, none of that would exist. Neil sees that although Brenda left Newark long ago, she is intimately connected to it. Perhaps the fact that she denies this connection is a big part of what ultimately pushes Neil away from her.